Most buildings are heated, cooled and ventilated using air forced through ducts and other confined air paths. It is common to filter the air upstream of a heating or cooling mechanism in order to remove particulate and gases from the air. Removal of these contaminants reduces wear and damage to the heating and cooling mechanisms, and it improves the air quality. For example, some contaminants are poisonous or harmful to the building's occupants. Others can damage or reduce the efficiency of the heating and cooling mechanisms, such as by building up to form a layer of insulation on a heat exchanger.
Conventionally, filtration of the air is accomplished by placing a filtration media in the air path so that air forced through the air path passes through the filter media. The filter media then removes contaminants by causing the air to pass through small, tortuous paths that strain particles, by electrostatic attraction and/or by chemical reaction. The filtration media is typically placed in a frame in the side walls of the air path, and that frame keeps the media from being carried along by the moving air. The air filter, which is the combination of the frame and the media, is removably mounted in the air path of the heating, cooling and ventilation system, and is replaced once its life span has been reached.
It is known that filter media can be placed in a planar configuration across the frame so that the plane of the media is perpendicular to the direction of air flow. It is also known that the effective surface area of the filter can be increased by “pleating” the media, which is forming alternating V-shaped bends (when viewed from the end of the media). Pleating media provides an overall thicker filter than with a planar media, but the gain in effective surface area substantially improves the filter's performance.
Conventional pleated filter media can be strong enough to support itself across the filter frame. However, some media are not strong enough to be self-supporting, so that a support structure must be used to prevent the filter from “blowing out”, which is where the force of the air pushes the media from its original position, possibly completely removing the media from the frame. Support structures used on pleated media include perforated material, such as aluminum, plastic, paperboard and steel. The support can be made of slit and expanded aluminum, for example, that is laminated to the downstream face of media that will be pleated and mounted in a frame.
Typically, support material for pleated media is positioned on the downstream face, which is the face of the media that encounters the air last as it flows through the filter. Downstream support for a pleated filter provides strength to the media without concern about the bonding strength of adhesives that attach the support to the media, which can be a problem with an upstream support.